The present invention relates to a method and a machine for the application of glue, in particular for the continuous production of glued objects.
Gluing is a wide spread method of joining diverse objects to form products. Perhaps in particular in connection with the production of laminated wood structures, efficient gluing is crucial in order to achieve high quality and economic production.
Oftentimes two-component glues are employed. Such glues comprises a first xe2x80x9cgluexe2x80x9d component and a second xe2x80x9chardenerxe2x80x9d component. These components must be adequately mixed before they are applied onto the surface of the object that is to be joined with another object to form a composite or laminated structure, such as a construction beam.
There are many known systems and methods for the mixing of glue and hardener, and for the subsequent application of the mixture. However, there are only two major types of machines for this purpose.
In the first type of machine glue and hardener is pumped from storage, mixed in a mixing device, from which it is pumped to an adjacent applicator machine. The application is carried out intermittently, i.e. it starts when the object enters and stopS when it exits the applicator station of the machine. When the next piece of material enters it begins applying glue mixture again. A position sensor such as a photo-cell indicates when the object enters the machine and thus triggers the start of application. The same position sensor indicates when the object leaves the machine, and triggers the interruption of glue application, after an appropriate time delay. It is also possible to use two position sensors.
In the second type of machine glue and hardener is first mixed in a mixing apparatus, and then mixture is circulated in an application machine. The mixture is fed to an applicator where it is expelled continuously. Thus, between the objects undergoing glue application the glue mixture will trickle down and be collected in a vessel containing circulating glue mixture. Thus, the glue that is collected in this way will ultimately be fed back into the system again.
In both types of machine (and in general for any known method of application) there is always some waste of glue mixture. The magnitude of the waste differs and depends on several factors, but the main factor affecting the waste is the time needed for the material onto which glue is to be applied, to pass through the applicator machine.
In the first type of machine the applicator will begin to expel or output the glue mixture slightly before the material enters the applicator, and will stop to output the mixture slightly after the material exits the applicator.
In the second type of machine, waste during manufacture occurs to the fact that the continuous supply of glue mixture will cause some mixture to adhere to the end surfaces of the material, where no glue should be applied. However, since in this machine type the mixture is circulated, the waste will instead be dependent on the so called xe2x80x9cpot lifexe2x80x9d of the glue mixture. The xe2x80x9cpot lifexe2x80x9d is the time a glue mixture (glue and hardener) can be used before the viscosity becomes too high to be applied, because of the polymerization process that takes place.
Therefore, if the pot life is short, it will become necessary to stop the production and clean the system from hardened glue, which causes costly stoppages in the manufacturing line, and also leads to considerable amounts of wasted glue.
As an example of the amount of glue that is wasted, consider the following example of manufacture of a laminated beam, where a machine of the first type is used. The beam consists of a number of lamellas, each lamella being 150 mm wide and 14 m long. The glue amount that is used is 400 g/m2, and the waste at start and stop is 15 g. In this case the waste will be 3.6%. If the lamella is shorter, say 3 m, which is not uncommon, the waste will be as much as 12.5%.
On a yearly basis the total waste of glue in a manufacturing facility for construction beams, can amount to 5-10 tons of glue.
Thus, there exists a clear need to improve the methods and apparatuses used for applying glue in the manufacture of laminated or composite objects, such as construction beams, window frames, doors, block-boards for furniture etc.
The object of the present invention is therefore to provide a method and an apparatus for the application of glue where the waste of glue is minimized and preferably eliminated. This object is achieved with the method defined in claim 1, and with the apparatus defined in claim 8.
According to the invention any glue mixture that is wasted, i.e. that is not applied to an object, will be collected in a collecting vessel or tank, where it is mixed with only one component of the mixture, preferably the glue component. The ratio of xe2x80x9csingle componentxe2x80x9d:xe2x80x9cwaste mixturexe2x80x9d is preferably larger than 100:20.
Thereby the speed of curing will be so much reduced that the presence of the hardener will not affect the viscosity of the glue to any significant degree. Thereby the need for interrupting the production for cleaning will be eliminated, and essentially no waste will occur.
In one embodiment the ratio is larger than 100:30, whereby adjustment of at least the hardener flow must be performed in order to maintain the desired mixing ratio.
By providing the day tank for e.g. glue component beneath the glue applicator the waste glue mixture is collected and mixed in with the glue component. Thereby it is possible to reuse the waste.